This invention relates generally to riding lawn mowers, and more particularly to the manner of generating, distributing, and controlling electrical power from an electrical generator driven by an internal combustion engine to power computer-controlled electric motors. The motors are used as the traction drives, blade drives, and lifting/tilting control of the mower deck in riding lawn mowers. An inverter provides power to on-board vehicle attachments and external electrical equipment, and may be inhibited during the running of the drive motors.
Riding lawn mowers exist today in several configurations and are purposefully built to meet the needs related to the tasks they are expected to perform. They require a power source that is typically directly or indirectly mechanically linked to the drive wheels for traction and may utilize a mechanical power take off connection provided for powering onboard attachments and externally attached devices.
Drive power trains have typically used drive axles, chain/sprocket drives, belt/sheave drives, manual gear-selection transmissions, hydrostatic transmissions, differential gears, and other mechanical parts in varying combinations. Steering and speed control techniques vary between the different types of mowers. Most use a mechanical differential in the drive train to balance the torque applied to the driven wheels so that the wheels can rotate at different speeds when they are required to make a turn. Some are capable of zero radius turning. The power sources for the mowers have been either battery-powered electric motors or internal combustion motors. Both of these sources have shortcomings when they are used separately in a drive system.
Undesirable features of battery-powered motor driven mowers have been the battery charge cycle, battery life, battery weight, battery cost, space required for the batteries, and low torque from the motors at low speed. Many tasks cannot be completed without the batteries having to be recharged due to the length of operational time required or due to the batteries not being fully recharged. Additionally, the charging time required can be excessive. Furthermore, the weight of the batteries also adds to the load on the drive and a large space is required on the mower for mounting the batteries. Thus, batteries have not been practical for real world applications, and this is especially true on the larger commercial mowers.
The internal combustion engine has features that detract from its use in directly driving a transmission and differential. Low output torque at low speed and decreasing torque beyond an optimum speed somewhere below maximum speed occurs in this engine. A typical engine will have a range of speeds up to 3300 RPM but torque efficiency will be maximized between 2500 and 2800 RPM. The loss of efficiency increases the thermal dissipation in the engine which causes fatiguing and failure of engine components. At low speeds, excessive vibration of the engine is also a problem. Continual operation of the internal combustion engine at its most efficient speed is desirable.
The blades on mowers are limited in the speed of the blade tips by the ANSI standards for mowers. The speed of the blades on existing mowers must be set at ninety percent or less of the maximum ANSI speed because the blade speed on a mower that has an all-mechanical system driven by an internal combustion engine will have a speed variation of plus/minus ten percent of the nominal set speed. On an electrically-driven blade that is electronically controlled, the blade speed can be very closely regulated. This precise speed regulation makes possible the operation of the blades at one hundred percent of the maximum ANSI speed.
Advances in electric generators and motors have increased their efficiencies to the 80-90% range and improved their reliability. The different types of electric motors are inverter-powered AC induction, brush-less DC, and switched reluctance motors. All of these types of motors have excellent controllability with state of the art controllers, which are of similar complexity. However, the prior art has failed to take advantage of these advances and provide improved riding mowers.
With the advent of solid-state power-switching devices such as MOSFETs (metal oxide semiconductor field effect transistors), IGBTs (insulated-gate bipolar transistors), and microprocessors, the electronic controls for generators and motors that were very complex and expensive in the past, have become economically practical. Today, the electric generator/motor drive provides the flexibility in control and the ruggedness in assembly needed for an electric motor-driven riding mower, especially one intended for commercial usage.
Portable electrical power sources have usually been either alternators or DC generators powered by rechargeable batteries or an internal combustion engine. Batteries are impractical to deliver enough power for a large horsepower mower or for its attachments. Both the generator and alternator require speed regulation of the engine to control their output. In the case of the alternator, its output frequency was determined by the speed of the driving engine. An inverter with electronic control on the output of a DC generator of the switched reluctance type can synthesize a sine wave power output with regulation of amplitude and frequency and a power efficiency near ninety percent. This improvement is not known to have been utilized on a riding mower.
Thus, what is needed is an apparatus and method for generating, distributing, and controlling electrical power to drive motors on the driven wheels, cutting blades, and the lifting/tilting of the mowing deck on a riding mower with output power available in a standard recognized format.
This invention relates to the apparatus and method of generating, distributing, and controlling electrical power to drive motors on the driven wheels, cutting blades and the lifting/tilting of the mowing deck on a riding mower. The mower drive system integrates 1) a high-efficiency switched reluctance electric generator/inverter driven by an internal combustion engine, 2) high-efficiency switched reluctance electric motors, 3) speed reduction gearboxes and 4) power controllers. The battery used for starting the internal combustion engine supplies power to the master controller and other low voltage circuitry.
An improved innovative mower drive system has been developed with an electric generator driven by an internal combustion engine and an electric motor that provides high output torque up to its base speed. The generator supplies electrical power through a motor controller to the motor/gearbox on each driven wheel, the lift/tilt control, and blade motor on each driven blade.
Thus, the present invention discloses an electric riding mower which integrates three sections: 1) an electric generator/inverter section driven by an internal combustion engine, 2) an operator/drive section with an electric motor driving each of the two wheels and 3) a mowing deck that has electric motors driving the cutting blades and a motor for raising the height/tilting of the deck. The generator is mechanically driven by the output shaft of an internal combustion engine to generate the electrical power for energizing the electric motors and the inverter. A computer in the master controller communicates with each of the controllers for the motors and the generator/inverter and processes the data necessary to co-ordinate the drives. The generator/inverter controller adjusts the output of the generator and inverter as an integral part of the mower or as a stand-alone power source. Each controller contains a control board and one or more power boards. The two wheel drives and the lifting/tilting drive of the mower deck each have individual controllers. The cutting blades on the mower deck have a common controller. The speed and torque of each of the motors in the drive system is controlled by its controller. The speed input signal for the wheel drive motors can be analog signals that come from sources such as a joystick, a potentiometer mounted on a steering wheel, dual sticks, a control panel, a foot pedal or remote signals from a remote control device. Position/speed detectors on each motor and the generator send signals back to their respective controllers for closed-loop control of the generator and of the motors. The generator supplies the DC voltage to the power board for each motor as commanded by its controller. The rotor of each wheel drive motor is connected to a gearbox for speed reduction and increased torque that is applied to the wheel mounted on the output shaft of the gearbox. With independent control of each wheel drive, the mower can make zero radius turns. The generator also supplies DC power to the input of an inverter that has an output to standard electric utility low voltage AC outlets that can be used to power auxiliary equipment when the drive motors are de-energized.